The distance travelled by the particle in the time 0 to 3 s is equal to the area shaded in the ure.
This is a right angled triangle with height `=6m/s and the base =3s`. The area is `1/2` (base)(height)`= 1/2xx(3s)(6m/s)=9m`. Thus, particle covered a distance of 9 m during the time 0 to 3. .